Neurofibromatosis is a genetic disorder characterized by tumors or neurofibromas that grow in the nervous system and under the skin. The disease occurs as two distinct types - neurofibromatosis 1 (NF1) and neurofibromatosis 2 (NF2) - with each type having a unique set of manifestations. NF1 occurs more commonly, in approximately 1 in 3000 births, whereas NF2 affects roughly 1 in 25,000 individuals.
Children with NF1 frequently develop orthopaedic problems. The two most common are spine deformities, particularly scoliosis - a curvature of the spine in a sideways or lateral orientation - and tibial dysplasia, which refers to failure of the tibia (shin bone) to develop appropriately. Patients with tibial dysplasia may also have a tendency to fracture and to develop pseudarthrosis, which refers to failure of the fracture to heal properly. Tibial dysplasia can also result in limb length discrepancy, where one leg is longer than the other. Other children with NF1 may develop overgrowth of a limb without any underlying bony abnormalities; in these cases, the underlying problem is soft tissue overgrowth.
In contrast to NF1, NF2 is associated with “acoustic neuromas” that may affect hearing and balance and frequently do not have any orthopaedic manifestations of their disease. As a result, this interview will focus on issues related only to NF1.
A diagnosis of NF1 is made in children with two or more of the following criteria:
Café au lait spots (left) and axillary freckling under the armpits (right)
In cases where diagnostic findings are not conclusive, but neurofibromatosis is suspected, the child should be closely monitored by a multidisciplinary team that includes a pediatric neurologist and a geneticist.
Children with NF1 who develop scoliosis may be diagnosed with either dystrophic (degenerative) or non-dystrophic forms of the condition. Non-dystrophic scoliosis resembles idiopathic adolescent scoliosis, with similar types of curves and curve patterns, according to Roger F. Widmann, MD, Chief of Pediatric Orthopaedic Surgery at Hospital for Special Surgery (HSS).
Treatment for non-dystrophic scoliosis can include bracing as well as surgery. Instrumentation is used to realign the spine and bone grafting is performed to help achieve fusion. While the health risks associated with non-dystrophic scoliosis are less pronounced, these curves can modulate into dystrophic scoliosis over time and therefore need to be monitored carefully until the child reaches maturity.
X-ray images of dystrophic (left) and non-dystrophic (right) scoliosis
“In cases of dystrophic scoliosis, we typically see shorter, more sharply angulated curves,” Dr. Widmann says. Characteristics of dystrophic scoliosis also include a number of bone abnormalities, including thinning of the ribs, significant rotation of the vertebrae, vertebral wedging, and erosion of the vertebrae by the spinal fluid, owing to the poor quality of the bone. Dystrophic scoliosis is also associated with kyphosis or a forward tilt of the spine, which can progress rapidly and may result in impingement of the spinal cord.
Because of the risks associated with this condition, orthopaedic surgeons usually recommend surgery at a smaller curve magnitude than with non-dystrophic scoliosis, that is, a curve of between 20 and 40 degrees. In contrast, in cases of idiopathic adolescent scoliosis, surgical intervention is most commonly reserved for cases in which the curve exceeds 50 degrees. Bracing is usually not an option in cases of dystrophic scoliosis.
Surgery to correct a dystrophic scoliosis curve can present special challenges, including the need to correct a kyphotic curve at the same time, Dr. Widmann explains. These surgeries also carry a risk of pseudoarthrosis or non-union. To help minimize this risk, Dr. Widmann and his colleagues at HSS use a graft from the patient’s iliac crest bone (part of the pelvis) to achieve the fusion, as opposed to the allograft often used in other scoliosis surgeries. Because of these complicating factors, more than one surgery may be required.
X-ray images of dystrophic (left) and non-dystrophic (right) scoliosis after surgery
Unlike the benign bowing of the legs that is not uncommon in healthy young children, the bowing of the tibia that is associated with neurofibromatosis has a distinctive orientation - an anterolateral bow - and may lead to fracture and tibial pseudarthrosis (non-union). In such cases, surgical intervention is recommended.
X-ray of an anterolateral bow in the tibia of a young patient, as seen in the leg on the right
While there is no cure for neurofibromatosis, careful monitoring and intervention for orthopedic and other complications can help improve the patient’s quality of life significantly.
Interview and summary by Nancy Novick